The distillation of alcohol from grains and the like produces dilute liquid solutions that are rich in energy producing materials referred to as Distillers Dried Grains and Solubles or "DDGS", which is the main by-product of alcohol production. As a by-product commodity, it is required that DDGS be produced and sold in a dry state or condition, for example as bulk feed to be used in animal husbandry. Characteristically and based for example upon corn, DDGS is 92% dry substance, 28-31% raw protein, 12-13% raw fat, 10% raw fibers, 5-6% ash, and 41-43% Nitrogen free extract. For example, in the manufacture of Ethanol, the following feed stocks can be used:
Wheat PA1 Corn PA1 Rice PA1 Sugar
Accordingly, the Peltier Freeze Concentration Process of this invention is not to be limited to the material processed, whether it be the purification of a primary fluid such as sea and/or brackish water, or other inorganic and organic solutions refining concentrations such as DDGS. For purpose of example, the approximate temperatures recited herein are for desalination of sea water.
The state of the art has employed the process of Absorption Freezing Vapor Compression for the concentration of sea water or brackish water. The process referred to is a vacuum freezing process in which the freezing is accomplished in a stirred tank crystallizer due to the evaporation of water vapor which in turn is absorbed in an adjacent chamber by a concentrated solution of sodium chloride (NaCl) diluted by the water vapor pumped to a compressor where it is concentrated to its original strength by vapor compression apparatus using a closed circuit. It is this vapor compression which is replaced in the present invention by the Peltier Multi-effect Process as it is hereinafter disclosed.
The generator means G or absorbent concentrator employed herein is a Peltier Effect Diffusion Still or PEDS, an electrically operated regenerator that drives water from the absorbent in solution therewith. In practice, Sodium Chloride, Calcium Chloride, or Lithium Bromide or the like is the absorbent. In general terms the generator is a thermoelectric still which includes a series of concentric endless walled sections which sections are closely spaced and between which a series of thermoelectric elements are positioned as heat pumps. Means are provided for the passage of distillate along the radially outer surface of each of the concentric sections. Water vapor is diffused from the thin film of liquid passing along the outer surface of each section, across a narrow gap or endless space to the inner surface of the next adjacent concentric section which serves as a condensing surface. The heat for the diffusion of the water vapor at a predetermined constant temperature is provided by thermoelectric heat pumping from the first concentric section relative to the axis of the apparatus in series radially outward to the most distant section from the axis. Diffusing water vapor is thus evaporated and condensed respectively from one section to another requiring only the current necessary to pass the heat flux from the inner to the outer surface. Means are provided for removing the distillate from the inner surface of the concentrator sections and the effluent strong absorbent water from the outer surface of the respective concentric sections. Reference is made to the structure of such Peltier Effect Concentric Still as it is disclosed in my U.S. Pat. No. 3,393,130 dated July 16, 1968, and in my subsequent U.S. Pat. Nos. 3,671,404 dated June 20, 1972 and 3,801,284 dated Apr. 2, 1974.
In order to achieve the temperature differential between the evaporating surface and condensing surface of the apparatus, a thermoelectric heat pump is utilized in the form of thermoelectric elements known to the art and described in texts as "Semiconductor Thermo-elements and Thermoelectric Cooling" by A. F. Ioffe, Infor Search, Ltd., 1957, and in various patents such as U.S. Pat. No. 2,959,017 Gilman, et al., "Heat Exchangers Employing Thermoelectric Elements for Heat Pumping", issued Nov. 9, 1960, and U.S. Pat. No. 2,978,875, Lackey, et al., Plural Stage Thermoelectric heat pump is a device utilizing Peltier phenomenon of heat absorption and heat dissipation at junctions between bodies having different thermomotive properties, which phenomenon occurs when electric current is passed through the bodies. A number of junctions are coupled and generally employed in a heat pump of this type, the couples being physically and electrically interconnected to form a thermoelectric array. A thermoelectric element of the type employed in connection with the present invention typically comprises an N-type thermoelectric element and a P-type thermoelectric element. The N- and P-type components are made from semiconductor materials used in thermocouples. An example of N-type material is an alloy of bismuth-telluride and bismuth-selenide having a formula of 75% Bi.sub.2 Te.sub.3 -25% Bi.sub.2 Se.sub.3. An example of P-type material is an alloy of bismuth-telluride and antimony-telluride having the formula 25% of Bi.sub.2 Te.sub.3 -75% Sb.sub.3 Te.sub.3. Semiconductive components including antimony and bismuth have been found suitable for use respectively as P- and N-type materials. Such materials or the formation of thermoelectric pumping elements are not claimed as novel per se in the present invention, but an example will be set forth of the type of such element employed in connection with the apparatus of the present invention.
In accordance with the present invention, a Peltier still simultaneously supplies a steam-jet refrigeration means R and an absorber-freezer means A, taking the latent heat associated with absorption and regenerating the absorbent fluid (NaCl) refrigerant to its original strength and producing high pressure steam. The concentration of sodium chloride permits operation at a temperature above the temperature level in the absorber section of the absorber-freezer means. The use of sodium chloride absorbent also eliminates contamination of the absorbent with the primary fluid being processed, especially when said primary fluid is sea water which is largely NaCl. It is to be understood that the absorbent fluid can vary and be chosen for its compatability with the primary fluid being processed, all as circumstances require.
It is a general object of this invention to usefully employ the effects produced by a Peltier still, namely the simultaneous production of pressurized steam and strengthened absorbent. Accordingly, these effects are advantageously employed in the combined operations of a steam-jet refrigeration means and of an absorber-freezer means. A primary absorbent fluid (NaCl) flows in a closed circuit between the Peltier still and the absorber section of the absorber-freezer means; and a secondary heat transfer fluid (NaCl) flows in a closed circuit between the steam-jet refrigeration means and the input of dilute feed solution into the freezer section of the absorber-freezer means. A feature is the complementary refrigeration of heat transfer fluid (NaCl) by the steam-jet refrigeration means and by the absorber section of the absorber-freezer means, pumped through parallel closed circuits and passed through a pre-cooler means C that lowers the temperature of the dilute feed solution near to freezing. Accordingly, the dilute feed solution is in optimum condition for processing in the freezer section of said absorber-freezer means.
The steam jet refrigeration means R employed herein is a fluid pressure apparatus that operates through the application thereto of primary high pressure steam used to energize an ejector that induces a secondary fluid in the form of vapor drawn from an evaporation chamber. The primary motive steam is expanded through a converging-diverging nozzle to velocities of the order of 1200 meters per second (4000 fps.). The corresponding nozzle pressure is very high, and the high velocity steam issuing from the nozzle entrains the water vapor leaving the suction-evaporation chamber, and the two streams merge in a mixing section that converges in the direction of flow. Such an arrangement is diagrammed in the drawings. Heat transfer fluid return is sprayed into the evaporator chamber and the chilled heat transfer fluid is withdrawn therefrom and utilized for heat transfer in a pre-cooler C as shown herein. The absorber and freezer are combined in the absorber-freezer means A which involves one vessel to eliminate the cumbersome equipment required to handle large volumes of low pressure fluids. The warmer absorbent prevents the build-up of ice on the upper surfaces of the freezer section, thus eliminating problems associated with prior art vacuum freezing processes. The dilute feed solution is agitated by rotating nozzles which throws said solution into the chamber thereby producing a large area for heat transfer. This same action is used to wash the walls of the freezer section so that ice does not accumulate thereon and so that the ice slurry drops to the bottom of the vessel chamber for discharge to the melter-washer means W.
A slurry of iced product is produced in the freezer section of the absorber-freezer means A, and which is drawn off and pumped to a melter-washer means W where the iced product portion thereof is scraped off the top of the wash column and falls into an annulus chamber that surrounds said column where it is melted by the exhaust steam from the aforesaid steam-jet refrigeration means. Discharge of the melt is at 34.degree. F. through a heat exchanger E2 absorbing heat from the incoming feed of dilute solution; while discharge of concentrated by-product solution, such as brine in the case of desalination, is at 26.degree. F. which is pumped off for further processing; for subsequent compaction into solid form when the product is DDGS as hereinabove stated.
Preliminary to conducting the process herein disclosed, the dilute feed solution is deaerated by means D wherein the oxygen content is lowered to about 1 ppm removing about 85% of the air contained therein. This reduces corrosion, and drastically reduces size and power consumption of the air vessel system that would be required if the air had to be removed from a pressure of 3.3 mm Hg in the freezer section of the absorber vs 18 mm Hg in the aerator means. (values are approximate)
It is an object of this invention to advantageously combine and relate the aforementioned means G, R, A, and W with related means as shown, whereby the aforesaid refining process is made possible as circumstances require. Operation of the generator means G is dependent upon a D.C. power supply and made effective by a feed water supply of weak absorbent continuously pumped thereto. Operation of the steam-jet refrigeration means R is dependent upon the high temperature-high pressure output of said generator, and relies upon the melter-washer means W and a condenser means Kl to reduce temperature and pressure before recirculation with the absorbent into the generator means G. And, the absorber-freezer means A is interdependent upon the output of each of said means G, R and a pre-cooler means C. The slurry output from the absorber-freezer means A is pumped into the wash column of the melter-washer means W where the product ice floats to the melter section and is skimmed off and subjected to the exhaust steam from the steam-jet refrigeration means R. The wash column and melter section of means W discharges product and by-product via separate discharge lines.
Two forms of invention are shown herein, the basic low temperature multi-effect system of FIG. 1 involving primary and secondary refrigerant fluids such as sodium chloride (NaCl), and a lower temperature vapor compression system of FIG. 12 which involves a third refrigerant fluid such as ammonia (NH.sub.3) in a cascading combination that lowers operational temperature in cases where the circumstances require it. It is an object therefore, to further lower product processing temperature by sharing the steam produced by the aforesaid Peltier generator means G in order to operate both the steam jet refrigeration means R and a closed circuit mechanical refrigeration means such as a turbine driven compression system with ammonia (NH.sub.3) as the refrigerant. In practice, part of the steam produced by the generator means G powers an expansion turbine T that operates a compressor means L, with heat of compression removed from the ammonia by the aforesaid secondary refrigerant (NaCl), and all of which chills and increases the subsequent cooling effect of the ammonia and eliminates the usual necessity of a cooling tower. Accordingly, the compressed ammonia is passed through a condenser where it is chilled by the brine circulation through the steam-jet refrigeration means, said chilled ammonia being discharged through an expansion valve E and passed through the pre-cooler C. Return of gaseous ammonia to the compressor is through a receiver S where it is further cooled by recirculating a portion thereof from the receiver and through the absorber section of the absorber-freezer means A. A continuous supply of coolant removes heat from the cascading third stage ammonia system, thereby providing a combined system of self contained configuration.